The invention relates to a synchronizing system for a private digital exchange ccnneoted to an integrated services digital network (ISDN).
The fundamental characteristics of ISDNs as defined by Recommendations of the International Telegraph and Consultative Committee (CCITT), and which are destined to replace earIier telepphone networks, are described in numerous publications, including, for example: "Le RNIS techniques et atouts" (Techniques and Advantages of ISDN) published in France in 1987 in the Teleccommunications Technical and Scientific Collection by a group of people under the name G. DICENET.
One of the objectives of such networks is to make it possible to provide telephone and other services using a limited set of types of connection and versatile user-network interface arrangements.
The connection of private exchange type user installations to an ISDN is thus provided for via "base" accesses having the reference TO.
A TO base access is designed to be two-way, with two symmetrical pairs of conductor wires each providing one-way transmission with a maximum data rate of 192 Kbits per second (Kb/s).
Two "B" channels each having a capacity of 64 Kb/s serve to convey information in digital form and transparently, and a "D" channel having a capacity of 16 Kb/s serves to transmit data in packet mode for services as well as signalling which is likewise digitized. The remaining 44 Kb/s are used for management functions, in particular for frame synchronization and for activating the access. When base accesses are remotely powered by the ISDN, the pcwer consumption of the network can be reduced by deactivating base accesses while they are temporarily unused.
With an exchange type user installation connected to an ISDN via at least one TO base access, a time base is provided in the installation in order to provide the necessary clock signals and synchronizing signals in the event of the corresponding signals from the ISDN being absent, it being understood that network signals take priority as soon as communications pass via at least one TO base access.